It is customary to have an alternate power source for an installation if a primary, commercial power source is lost or becomes unreliable. The alternate power source may take the form of, for example, a generator driven by an internal combustion engine. When a transformation from the commercial power source to the alternate source is required, a transfer switch is used to disconnect the commercial power source and thereafter to connect the alternate power source to the load. The transfer switch conventionally provides an open, or break-before-make, transition between the two power sources. More specifically, the transfer switch disconnects the initial commercial power source before connecting the alternate power source to the load. The open transition switching avoids any potential surges that may occur as a result of the two unsynchronized sources applying power to the load simultaneously.
Open transition transfer switches to accommodate the precise application as described above are well known in the art. For instance, transfer switches utilizing a pair of circuit breakers and a mechanism for operating the two circuit breakers in opposition are a product of former patents. U.S. Pat. No. 3,778,633 to DeVisser, et al., issued on Dec. 11, 1973, entitled "Automatic Electric Power Source Transfer Apparatus," discloses a device that includes two circuit breakers mounted end-to-end and operated in opposition by a ganging member driven rectilinearly by a screw shaft engaging a traveling nut connected to the ganging member. Another transfer switch of this type uses two side-by-side circuit breakers with handles. The handles are engaged by clevises attached to a lever arm that is rotated about a pivot axis between the two circuit breakers. In still another transfer switch using two side-by-side circuit breakers, a plurality of slides engaging the circuit breaker handles are driven in opposition by gears having an eccentric pin to engage a camming surface on the slide.
The previously-described transfer switches provide open transition operation and thus momentarily interrupt power to the load. However, the open transition switch, where there is absolute and unabridged interruption of power to the load for a period of time, is unacceptable in certain situations. For example, a digital computer will lose the contents of its volatile memory if the power source is severed for even a very short interval. In yet another example, a central office with equipment serving a telephone network cannot be deprived of power for even an instant, or service is lost throughout the network.
To accommodate such critical applications, uninterruptible power supplies ("UPSs") have been developed to provide continuous power to the computer or central office equipment should the commercial power source be lost. One illustration of an uninterruptible power supply is a battery that is ordinarily charged by a charging device connected to the commercial power system. When commercial power source is lost, the battery provides direct current ("DC") power to the load and an inverter coupled to and fed by the battery provides alternating current ("AC") power to the load. Conventionally, a special static switch, located in proximity to the inverter, provides a rapid open transfer between the commercial power source and the inverter. The resulting interruption of power only lasts for a few milliseconds, thereby preserving the integrity of the power to the load and not adversely affecting the digital computer, central office equipment or other critical load.
Occasionally, the uninterruptible power supply, including the inverter, must be removed from the power system to perform routine maintenance, for example. In such cases, a bypassing mechanism should be provided that is capable of disengaging the uninterruptible power supply from the power system without provoking an interruption of the power to the load. Prior art bypass switches, such as U.S. Pat. No. 5,081,367 to Smith, et al., issued on Jan. 14, 1992, and entitled "Electric Power System with Maintenance Bypass for Uninterruptible Power Supply Using Closed Transition Operation," disclose maintenance bypass switches for disconnecting an interruptable power supply.
The maintenance bypass switch of Smith, et al. provides a first switch in series with the uninterruptible power supply and a second switch that shunts the uninterruptible power supply and the first switch. The first and second switches are circuit breakers having handles which toggle between open and closed positions. The maintenance bypass switch includes an operator that operates the two circuit breakers in opposition with a closed transition so that one switch is always closed. An interlock prevents operation of the maintenance bypass switch when the alternate power source of the uninterruptible power supply is selected by the static transfer switch. An isolation switch between the uninterruptible power supply and the commercial power source and also shunted by the second switch of the maintenance bypass switch is open together with the first switch of the maintenance bypass switch when the uninterruptible power supply is to be isolated for maintenance. However, the prior art bypass switches, including Smith, et al., are deficient in some significant respects.
First, the prior art bypass switches only provide single line switching; even in the bypass mode of operation, a current always flows through an unswitched neutral line. Therefore, a person servicing an uninterruptible power supply coupled to the prior art bypass switches risk injury if the neutral line carries an electrical current.
Second, the transfer and restore procedures of the prior art bypass switches are cumbersome and unforgiving. To enter the bypass mode of operation, a user must meticulously follow the transfer sequence instructions. This involves simultaneously switching both the maintenance bypass switch (for AC bypass) and a separate inverter selector switch (for DC disconnect). If the user does not toggle the switches precisely at the same time, the bypass mode will not be achieved successfully leading to a situation where the two sources may be paralleled for an extended period of time or, worse yet, the load will decouple from the power source. Again, in the critical applications for which the uninterruptible power supply is designed, loss of power for even an instant is unacceptable.
Third, while Smith, et al. addresses some of the aforementioned deficiencies, Smith, et al. still does not provide a means for switching the DC source (i.e. battery) which may be hazardous depending on the voltage and charge capacity of the battery. Finally, Smith, et al. does not provide AC isolation between the uninterruptible power supply and the commercial power source in a single operation.
Accordingly, what is needed in the art is a maintenance bypass switch that provides more reliable isolation of both AC and DC power and simplifies entry into the bypass mode of operation by integrating the switching of both AC and DC power into a single maintenance bypass switch operable by a single selector.